The Environmental Protection Agency has issued regulations prohibiting the discharge of metals in process waste water streams. Many industries producing metal bearing liquid effluents simply do not comply with these regulations and illegally discharge untreated waste effluents into streams and sewers. The two major alternative methods for complying with metal discharge regulations by these industries are the storage and transport of untreated, unconcentrated wastes to hazardous waste disposal sites, or, alternatively, on-site treatment. Storage and transport of waste is very expensive for all but the very small volume waste producers.
On-site treatment is an effective means for ensuring compliance with disposal regulations. The most viable chemical techniques for on-site treatment of metal bearing effluents include electrolytic deposition, metallic replacement, ion exchange, chemical reduction and chemical precipitation. While, electrolytic deposition, metallic replacement, ion exchange and chemical reduction are all reasonably effective, chemical precipitation is believed to be the most effective method for metal removal from waste water effluents. Two main chemical precipitation methods are known for removing heavy metals from waste water--hydroxide precipitation and sulfide precipitation. Hydroxide precipitation has limitations, however, due to high solubility and amphoteric properties of metal hydroxides. In addition, the technique is not effective in the presence of chelating agents, which are commonly used in metal finishing operations. See, Treatment of Metal Containing Waste Water with Calcium Sulfide, AICHE Symposium Series No. 209, Vol. 77. Sulfide precipitation is an alternate method which does not have the shortcomings associated with hydroxide precipitation. Three major problems, however, exist with sulfide precipitation. These are the necessity to control excess sulfide ion; the necessity to control pH to avoid the production of toxic and noxious hydrogen sulfide gas and the problems associated with the filtration of the very fine metal sulfide particles.
Despite these problems, sulfide precipitation by manual and semi-automatic means has been shown to be a highly effective method of metal recovery from processing effluents. Nevertheless, the method has not been widely used due to certain hazards and complexities which render it commercially impractical. The sulfide precipitation method of metal recovery involves a sequence of dynamic chemical reactions and mechanical processes which, when under manual or semi-automatic control, require repeated intervention by an attentive operator. The interventions required by the operator require a significant degree of skill and training. Moreover, the varying methods of metal precipitation uniformly require some degree of exposure to strong chemical reagents, which by their caustic, explosive, or reactive nature may endanger the safety of the operator. In addition, sulfide precipitation methods now in use generally result in final sludge products of less than 10% solids which are difficult to handle, store and transport. Potential liberation of such material also presents an additional hazard due to the toxic or reactive nature of metallic or chemical components.
Chemical precipitation methods now in use also generally employ a chemical fume-hood or open venting system against the possible liberation of hydrogen sulfide, ammonia, or other gases from the process which may adversely affect exposed material, processes, and personnel.
While the sulfide precipitation method of metal removal and recovery has previously been subject to remote monitoring and/or semi-automatic control, no comprehensive control system is known with the capability to continuously stage, monitor and respond to all of the control aspects of the method, so as to render the method practical for commercial application. It would be desirable to provide an improved apparatus for recovery of unwanted metals from waste water effluents by chemical precipitation which is automatic and which, therefore, avoids the serious disadvantages referred to above.
In addition, chemical precipitation methods of metal recovery now typically employ either a settling system or a cartridge filter media for final removal of metal precipitates. These settling systems and cartridge filter media also suffer from serious disadvantages. Precipitate settling systems require large amounts of space and deliver flowable sludges as the final end product. Cartridge filter media now in use are too slow for commercial applications and require repeated replacement of the expensive cartridge filter media. It would be desirable to provide an automatic metal recovery apparatus as referred to above, which also avoids the serious disadvantages resulting from the use of settling system or cartridge filter media.